The manufacture of semiconductor devices and integrated circuits involves the blanket and selective deposition and blanket and selective removal of many layers of conductive, insulating and semi-conductive material on substrates that are usually in the form of silicon wafers. Important processes for such deposition and removal of material include physical vapor depositing and etching processes. Sputtering is one commonly used mechanism for providing material for coating and removing material in such physical vapor processes. In a conventional sputter deposition system, a target is sputtered and the sputtered material forms a thin coating or film on a substrate. In a conventional sputter etch system, material is removed from a surface such as a target or a substrate.
Semiconductor manufacture processes typically include forming a series of metal interconnect film stacks on a wafer, then applying a photo-resist pattern followed by reactive etching processes rendered selective by the pattern. Once patterned and selectively etched, a subsequent stack of conductive layers is applied to the wafer. The lowermost of these conductive layers is usually a reactive elemental metal such as titanium, chromium or tantalum, but may also be a metal nitride, silicide or alloy. One function of this lowermost metallization layer is to form a bond or contact with an exposed conductive layer, such as silicon or metal, at the bottom of a contact hole in the underlying insulator. The bond serves to form the initial film portion of a conductive path between the underlying layer and the conductor of a new layer of the new stack.
It is usually necessary to clean the wafer of native oxides and other contaminants that may form before applying the metallization layer or to otherwise condition the surface of the wafer before coating. Contamination interferes with application of the metallization layer and results in degraded conductivity between the contact and the metallization layer. However, standard approaches to removing contaminants, such as subjecting the wafer to a thorough cleaning with an inductively coupled plasma (ICP) or other soft sputter etch step immediately before initiating metallization, are not entirely satisfactory. This is due at least in part to the damage of the underlying device structure, either by the mechanical sputtering action or by the accumulation of charge, or to the incomplete removal of contaminants when chemical removal methods cannot be used or are otherwise absent.
In a conventional sputter deposition arrangement, a target and substrate are located within one or more processing chambers where the sputtering process is performed. The target includes a back surface and a concave shaped front, or sputtering, surface. The sputtering surface provides target material for forming a thin film on the substrate during the sputtering process. The back surface is secured to a sputtering cathode which serves to cool the target during the sputtering process. The substrate is removably secured to a support fixture adjacent an outer edge of the substrate and is positioned a predetermined distance from the sputtering surface, thus forming a gap between the substrate and the sputtering surface.
For sputter deposition of a material, a process gas such as argon is introduced into the processing chamber and maintained at a vacuum level suitable for sputtering. A high DC or AC voltage is then applied to the cathode and target to form a plasma discharge having positively charged argon ions which bombard the negatively charged sputtering surface. This causes target material to be removed from the sputtering surface and initiates a deposition process wherein some of the target material is deposited onto the substrate to form a film. Typically, the deposition process may require between 5 seconds and 5 minutes to complete. The substrate may be held in a stationary position relative to the sputtering surface during the deposition process or may be slowly scanned in a direction parallel to the sputtering surface.
The cathode may include a main magnet for concentrating the plasma and controlling the shape and relative intensity of the plasma over various locations on the sputtering surface. In addition, the main magnet may be adapted to rotate about a rotation axis or otherwise move relative to the target. A main magnet may also be positioned adjacent to a substrate to control sputter etching of the substrate. The main magnet is typically configured to create a continuous closed magnetic tunnel having a predetermined shape which may include a plurality of lobe portions, each having an outer peripheral lobe section located adjacent to the peripheral wall of the target. Rotation of the main magnet about the axis causes a corresponding rotation of the magnetic tunnel relative to the sputtering surface. This controls the plasma discharge so as to cause removal of target material in a symmetric pattern from the sputtering surface.
With a sputtering target, rotation controls the plasma so as to form concentric grooves such as primary, secondary and tertiary concentric grooves, each having respective diameters symmetrically formed about a center area of the sputtering surface. The primary groove has the largest diameter and is positioned adjacent to the peripheral wall. The tertiary groove has the smallest diameter and is positioned around the center area, and the secondary groove has an intermediate diameter and is thus positioned between the primary and tertiary grooves. Typically, the primary groove is formed deeper, and has a greater circumference, than either the secondary or tertiary grooves. This indicates that a greater amount of target material is eroded to form the primary groove. Therefore, formation of the primary groove provides a substantial portion of the material used to form a film, which thus has a substantial effect on overall film uniformity on the substrate. Further, the erosion of a substantial amount of material near the peripheral wall also improves the capability of providing a uniform film thickness in areas near the outer edge of the substrate.
It is desirable that a film formed on the substrate have a highly uniform thickness, to .ltoreq. about .+-.5% and preferably to .ltoreq. about .+-.1% for the thickest and thinnest areas of the film. Several factors affect the ability to produce a film having a highly uniform thickness. These factors include the geometrical relationship between the target and substrate, the design of the cathode and the erosion pattern of material removed from the sputtering surface resulting from the shape of the magnetic tunnel.
The main magnet generates a main magnetic field and serves to control the shape and intensity of the plasma discharge in order to ultimately form the primary groove. The primary groove includes a pair of walls, each of which extends gradually deeper into the target to meet at, and, thus define the deepest portion of the primary groove at a groove center. Further, the groove center is caused to be positioned a first distance from the peripheral wall and within a predetermined area of the sputtering surface.
Preferably, in sputter coating, rotation of the main magnet causes the formation of a symmetrical film on a substrate. The deposition profile would then represent overall thickness uniformity along any radius extending in any direction on a substrate. Further, any undesirable non-uniformities existing in a symmetrical film would also be symmetrical. Such symmetrical non-uniformities can be reduced by techniques such as changing the distance between the sputtering surface and the substrate or by modifying the erosion profile through adjustment of the shape of the magnetic tunnel.
However, factors exist which frequently cause the formation of asymmetrical non-uniformities. These factors include asymmetrical system related conditions such as the presence of nearby structures which may distort the shape of the plasma discharge, other cathodes located in nearby processing chambers, or the existence of flow and pressure gradients. These factors can distort the motion of ions and particles moving toward the surface being sputtered. In coating, this can mean that material that is sputtered or otherwise emitted from a source in a symmetrical pattern about an axis of the substrate will arrive at the surface of the substrate in a non-symmetrical distribution. In sputter etching, this can mean that the ions or other particles bombarding a surface of a substrate, even if emitted in a symmetrical pattern around an axis of the substrate, strike the substrate in a non-uniform distribution or at non-uniform angles around the axis. These factors can also affect the distribution of ions bombarding a sputtering target. As a result, if these factors are not taken into account and dealt with, they can cause non-uniformities in the coatings being applied to a substrate or in the removal of material from, or the conditioning of, a substrate or other surface from which material is being removed. Techniques for correcting symmetrical non-uniformities, such as changing the distance between the sputtering surface and the substrate or adjusting the shape of the magnetic tunnel, however, are not effective in reducing asymmetrical non-uniformities to an acceptable amount.
Thus, there is a need for an effective and inexpensive process that prevents contaminants from interfering with the metallization of surfaces at which contacts on the lowermost layer of a stack or other interconnects are to be formed. There is also a need to control physical processes to overcome the effects of factors that affect the uniformity of sputter deposition of material to reduce non-uniformities in a thickness of a deposited film.